Underlying mechanism of shear-banding in soft glasses of charged colloidal rods with orientational domains

D Parisi,K Kang,J K G Dhont,D Vlassopoulos,H Kriegs

doi:10.1122/8.0000400

Abstract

Soft glasses of colloidal rods (fd-virus particles) with orientational domains were recently shown to exhibit inhomogeneous flow profiles [Dhont et al., Phys. Rev. Fluids 2, 043301 (2017)]: fracture and accompanied plug flow at small shear rates, which transits to gradient shear-banding on increasing the shear rate, while a uniform flow profile develops at sufficiently high shear rates. These flow profiles coexist with Taylor-vorticity bands. The texture of such glasses under flow conditions consists of domains with varying orientations. The observed gradient shear-banding was solely attributed to the strong shear thinning behavior of the material inside the domains (henceforth abbreviated as domain-interior), without considering the texture stress that is due to interactions between the glassy domains. Here, we present new experiments on the shear-banding transition to assess the role played by the texture stress in comparison to the domain-interior stress. For a large concentration, well into the glassy state, it is found that both texture stress and domain-interior stress contribute significantly to the gradient shear-banding transition in the shear-rate region where it occurs. On the other hand, for a small concentration close to the glass-transition concentration, the domains are shown to coalesce within the shear-rate range where gradient shear-banding is observed. As a result, the texture stress diminishes and the domain-interior stress increases upon coalescence, leading to a stress plateau. Thus, a subtle interplay exists between the stresses arising from the structural order on two widely separated length scales from interactions between domains and from the rod-rod interactions within the domain-interior for both concentrations.

Highlights

IntroductionThe texture stress that includes all these contributions is shown, in [50], to be very small for fd-virus suspensions at much higher ionic strengths, where no rod-glass transition occurs
Since the local orientation couples with the stress, the flow velocity varies from one band to the other but in a less pronounced manner compared to the order parameter
We have performed velocity profile and shear rheology measurements which, complemented by optical microscopy imaging from the literature, elucidate the underlying complex mechanism for the existence of nonuniform flow profiles in soft glasses of long and thin, charged colloidal rods [51,52]

Summary

Introduction

The texture stress that includes all these contributions is shown, in [50], to be very small for fd-virus suspensions at much higher ionic strengths, where no rod-glass transition occurs. In the present study, we find that the domain-interior stress is of the same order of magnitude as the orientational-texture stress where tumbling and wagging are arrested and where, most probably, the degree of nematic alignment will not be significantly affected by the flow. At small applied shear rates, fracture and plug flow were detected, at an intermediate shear rate, there was a seemingly sharp transition to gradient shear-banding, while at high shear rates, a linear flow profile prevailed (see Fig. 2, which will be discussed in more detail below) All these flow profiles coexisted with Taylor-vorticity bands with a spatial extent that was much larger than the average domain size. The observed gradient shear-banding is attributed, in [51], to strong shear thinning behavior of the domain-interior once its corresponding yield stress was surpassed

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Conclusion